1. Field of the Invention
The present invention relates to an electronic device in which an electronic element such as a quartz crystal vibrator is accommodated in a package, an oscillator using the same, and a method for manufacturing an electronic device.
2. Description of the Related Art
Surface mount type electronic devices have hitherto been frequently used in cellular phones and portable information terminals. Above all, in a quartz crystal vibrator, MEMS, a gyroscope, an acceleration sensor, and the like, a hollow cavity is formed in the inside of a package, and an electronic element such as a quartz crystal vibrator and MEMS is enclosed in this cavity. A glass material is used as the package. For example, an electronic element is mounted on a glass substrate, and a glass cover is bonded thereon by means of anodic bonding, whereby the electronic element is sealed hermetically. The anodic bonding between glasses brings about such advantages as high airtightness and inexpensiveness.
FIG. 6 is a cross-sectional view of an electronic device of this type (FIG. 1 of JP-A-2011-155506 (Patent Document 1)). An electronic device 101 includes a base 110, an electronic component 140 mounted on the base 110, and a cap 150 bonded to the base 110 while accommodating the electronic component 140 therein. In the base 110, a through electrode 121 penetrating in a plate thickness direction, a first metal film 122 electrically connected to the through electrode 121, and a circuit pattern 130 and a second metal film 123 each electrically connecting the through electrode 121 and the electronic component 140 to each other are formed. An external electrode 160 composed of a metal film is formed in the outside of the first metal film 122.
Here, an iron-nickel based alloy is used for the through electrode 121. Gold which is formed by an electroless plating method is used for the first metal film 122. In addition, a non-illustrated low-melting point glass is used between the through electrode 121 and the base 110, thereby enhancing the airtightness by means of heat welding. When it is intended to conduct heat welding using a low-melting point glass, thereby enhancing the airtightness between the through electrode 121 and the base 110, an oxide film is formed on an end face of the through electrode 121, so that the conductivity with other metal is lowered. Then, the oxidation of the through electrode 121 is prevented from occurring by removing the oxide film formed at the time of heat welding of the through electrode 121 and then forming the first metal film 122 or the second metal film 123 on the end face of the through electrode 121.
An iron-nickel based alloy is used as the through electrode 121, and a gold thin film is used as the first metal film 122 for preventing the oxidation of the through electrode 121 from occurring. A difference in ionization tendency between the iron-nickel based alloy and gold is so large that when moisture or the like is attached between the through electrode 121 and the first metal film 122, the through electrode 121 is corroded due to a battery effect, resulting in causing a lowering of conductivity. In addition, in Patent Document 1, a low-melting point glass is used between the through electrode 121 and the base 110, and a gold thin film of the first metal film 122 is formed on the end face of the through electrode 121 by an electroless plating method. Since it is difficult to form a gold thin film on the low-melting point glass by the electroless plating method, a boundary portion between the through electrode 121 and the first metal film 122 is exposed, so that the corrosion is much more advanced.